Automobiles generally have one of two types of suspension systems: solid axle suspension or independent suspension. In a solid axle suspension system, opposing wheels of the automobile are mechanically linked to each other with a solid connection (shaft, beam, etc). Shock absorbers and links connect the solid shaft to the chassis of the automobile. In an independent suspension system, each wheel is individually connected to the chassis with its own shock absorbers and links.
Referring to FIG. 1, a solid axle suspension system is shown. The suspension has one clear characteristic where the left and right wheels are connected by one rigid element. When the wheels in the suspension are not powered (motor, engine, etc), as in the case of a two wheel drive vehicle with these being the non-powered wheels, the connecting element is usually an I-shaped beam. The suspension may be referred to as Beam axle. When the wheels in the suspension are powered (motor, engine, etc), the connecting element is usually a hollow tube housing a drive shaft with a differential located somewhere along the shaft. Some of the powered solid axles do not include the differential on the axle as in the de Dion tube, and some do not have a differential at all, as in many all-terrain vehicles (ATVs).
Solid axle suspension systems have several benefits, including: simple construction and manufacturing; can be very strong, and therefore used mainly in industrial vehicles and off-road vehicles; camber control during cornering; and has two degrees of freedom, one is the roll 102 about a longitudinal axis 104 or any other longitudinal axis of the vehicle. The second degree of freedom is the roll 106 about the lateral axis 108. In some solid axle systems, the connecting links 110 are in the form of a four-bar mechanism and the axis of rotation 108 is made much farther forward, which enables the wheels to travel almost straight up and down. Solid axles also have the advantage that forces and displacements are transferred from one wheel to the other because of the solid connection between them. Given these advantages, the vehicle possesses ground articulation capabilities that are hard to match by other suspension systems, and is particularly useful in specialty vehicles such as rock crawlers.
On the other hand, solid axle suspensions systems have a number of drawbacks. The systems are heavy; transfer the forces and displacements from one wheel to the other because of the solid connection between them; bump steer can be a problem; requires a large space to accommodate the size of the suspension; and lateral control is a problem in some cases. However, ride quality is probably the biggest issue with solid axles. Because of the large size and weight, the suspension is hard to move (conservation of momentum). And once it is moving, it is very hard to stop. At speed, on uneven terrain, it feels like going through bumps instead of going over them, and once the bump is long gone, the vehicle is still rocking up and down. This not only effects the ride quality, but the general limits of the capabilities of the entire vehicle such as slower safe top speeds and cornering.
Referring to FIG. 2, an independent suspension system is shown. An independent suspension system has one clear characteristic, which is that the left and right wheels are not directly connected to each other. There are many kinds of independent suspensions, such as: Double A-arm, McPherson Strut, Multi link, Trailing arm, etc. The wheels in the suspension maybe powered or not.
Independent suspension systems have several advantages, including: small size; light; can be very adjustable as far as camber, camber gain, caster, toe-in/out, and positioning of roll center; can have very good lateral control and no flexing such as in double A-arm; and each wheel can react to a given excitation independently from the other. Given these advantages, an independent suspension can react quickly to a bump in the road and quickly recover, which will keep the wheels of the vehicle on the ground and not constantly jumping up and down as in the solid axle suspension, especially at higher speeds. This makes the vehicle perform better since it is not losing energy while the wheels are spinning in the air. The chassis can remain fairly parallel to the ground and ride quality is gained. Cornering can be improved with the right adjustments, and so the overall performance is enhanced.
On the other hand, independent suspensions also have several drawbacks, including: only one degree of freedom 112 per wheel; the wheels are independent of one another and may be oblivious to one another; and in some cases where camber gain is designed into the suspension, the wheels will not stay parallel to each other under high loads, such as when the vehicle is carrying a large weight, tows a heavy load, which in turn causes a loss in traction. Given these disadvantages, an independent suspension will perform poorly on a vehicle that requires high loading capabilities or requires the ability to scale large obstacles.
A primary distinction between the independent suspension 114 and the solid axle suspension 122 is illustrated in FIG. 3, where a vehicle is shown passing an obstacle 120. The vehicle with an independent suspension 114 can be driven to a large body roll 116 in comparison to the roll 118 in a similar vehicle passing a similar obstacle 120 with a solid axle suspension 122. That is because the independent suspension does not have any control over the deflection of the suspension other than the spring-damper system, whereas in the solid axle suspension 122 the left and right wheels help control one another by being rigidly connected to one another.
Thus, other than the mechanical differences, the two types of suspension systems provide the automobiles with different capabilities in tackling difficult terrain. Referring to FIGS. 1-3, the solid axle suspension system 122 is preferred when driving over rocks or other large obstacles. Also, the solid axle suspension 122 is better equipped to articulate rocky terrain since it possesses more degrees of freedom per wheel than an independent suspension 114. The wheels are directly connected to one another and react to one another.
When driving on sand dunes, gravel roads, and other similar terrain, independent suspension systems are preferred. Because each wheel is individually and independently linked to the chassis, each wheel is better able to maintain contact with an uneven and ever-changing terrain. Accordingly, independent suspension systems 114 provide the automobile with a better ability to trace the curvature of the road. Also, independent suspension systems 114 can maintain the chassis generally parallel to the surface. Another big advantage of an independent suspension 114 over a solid axle suspension 122 is that the unsprung weight of the vehicle (wheels, brakes, suspension links, shocks and dampers) is much lower. Solid axle suspensions 122 also carry with them the differential and the solid bar between the wheels, which in most cases adds a few hundred pounds to the total weight of the suspension. This makes the solid axle harder to move, and harder to stop from moving (i.e., the larger the mass, the higher the momentum).
However, there is no suspension system that provides both the advantages of a solid axle suspension system 122 and an independent suspension system 114. An independent suspension automobile traveling over sand dunes, and gravel roads might encounter rocks or other obstacles that require a solid axle suspension articulation, and not be able to continue its journey. Thus, there is a need in the art for such a suspension system that provides both modes of suspension on the same vehicle, namely both solid axle suspension and independent suspension.